1. Field of the Invention
The present invention relates to a drill bit which may be used for deep hole drilling. More particularly, the drill bit includes at least one passage for through coolant, and is carbide tipped for use on a gun drilling machine, or most manual or computer controlled machine tools such as lathes, and/or milling and boring machines with stationary and/or rotary toolholders.
2. Description of the Related Art
Drilling deep holes efficiently has always been problematic in the metalworking industry. Prior art methods include the use of long twist drills of the type disclosed in U.S. Pat. No. 3,548,688. These drills must be retracted many times during the drilling process to remove chips formed as the bore is being produced.
Drill bits that provide for a coolant feeding use coolant under pressure to flush away the chips and have been found to be more effective, such as that disclosed in U.S. Pat. No. 2,954,712.
Both of the drill types mentioned above, being of steel construction, are limited to use with softer materials as harder materials severely limit tool life due to rapid wear of the tip. Furthermore, deep twist drills produce holes with inaccurate bore diameters and poor straightness and finish quality. Although the diameter tolerance of the hole may be flexible in certain applications, the prior art long twist drills may not even produce holes which meet such allowances.
Another alternative is the carbide tipped, single-flute gun drill as exemplified in U.S. Pat. No. 2,325,535. The diameter of the hole produced is often within a much better tolerance than required, due to the self-piloting and burnishing action of the tip. The single-fluted drill, formerly used exclusively on specialty built gundrilling machines, is now finding favorable applications on standard machine tools such as lathes, milling and boring machines, either manual or computer (CNC) controlled, with through tool, high pressure coolant, or air pressure and lubricant combined as a spraymist.
The single-flute gun drill method, although somewhat slow, remains as the best choice for deep holes with tight tolerances. Where hole tolerance is more flexible, however, it is desirable to provide a carbide tipped, coolant feed drill capable of drilling a wide variety of materials at a more productive feed rate.
This need has been partially addressed by two-flute, carbide tipped, coolant feed drills presently known in the art. There are currently several varieties on the market, each having similar construction with straight or helical flutes. Variants designed for deep holes all have flutes that are either milled from steel rod or central crimped tubing.
Most of the prior art two-flute drills provide a carbide tip with a thick web between the two cutting surfaces which leaves very limited clearance for chips, rendering their use to only the most free chipping ferrous and non-ferrous materials, i.e., gray cast iron, brass, or cast aluminum.
Additionally, various configurations have been designed to matingly fit the carbide tip to the distal end of the drill shaft. The contact area of these designs, however, are such that forces exerted on the brazed joint during the drilling process are concentrated on the braze material, which can result in weakening of the joint to cause separation of the shaft from the tip when used in materials which are harder than those noted above.
For example, in U.S. Pat. No. 4,664,567, and as shown in FIGS. 12 and 13 of the present disclosure, the contact surfaces between the drill tip and the drive shaft form a "V" shaped profile, wherein the planar axis of the "V" is in line with the crimp of the tube. This design produces a torque force concentrated on the braze material and thus relies on the strength of the braze material to maintain contact at the joint.
Any attempt to use the prior art carbide-tipped drills on materials which do not chip easily or on harder materials such as steel, for example, will result in failure due to heavy loading of the tip, and/or failure due to insufficient clearance in the flutes for the larger sized chips which are produced when drilling certain types of material, e.g., steel. These problems are compounded where the drill length is long and/or the operating conditions are less than ideal, i.e., worn or dull drill tip, coolant pressure too low, speed and/or feed too high, etc.
Carbide drills with more efficient grinds are generally limited to shorter length tools, typically those having a shaft length less than ten times the tip diameter (longer lengths become very costly in solid shaft construction). Even successful results obtained using shorter length drills are often achieved due to the physical strength of the drill assembly allowing for the mis-application of one or more operating parameters, i.e., less than rigid set-up, incorrect speed or feed, low coolant pressure, improper drill start, etc. When the drill shaft length is greater than ten times the tip diameter, the designs of the two-flute drills presently known in the art are so limiting that all operating conditions have to be precise to ensure a satisfactory result.